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Raising collar ties

  • 14-07-2013 1:00pm
    #1
    papa john
    Registered Users, Registered Users 2 Posts: 17 ✭


    Hi all
    I would like to raise collar ties in a roof for extra headroom.They are currently at 6ft above the joists with a further 3 feet up to the ridge board but i would like to raise them by at least 12 inches for extra headroom.The joists are 9x2 and span 15ft and are tied into the walls but dont sit on the wall plate.The pitch of the roof is 40 degrees. I know collar ties prevent roof uplift and also stop the rafters from seperating but am finding it hard to get good information on raising collar ties and when you can and cannot raise them. Is the joists i have good enough to stop the walls spreading meaning the collar ties are of less importance?


Comments

  • #2
    Rabbo
    Registered Users, Registered Users 2 Posts: 637 ✭✭✭Rabbo


    You really need to get an engineer to look at it.

    Most structural problems with roofs in 1.5 storey/dormer houses are due to roof spread- i.e. rafters which are not properly supported pushing out external walls.

    There are many variables- it would be very foolish to proceed without an engineers input.


  • #3
    chris445
    Registered Users, Registered Users 2 Posts: 239 ✭✭chris445


    If I'm reading this corrctly and your ceiling joists aren't sitting on the wallplate then they can't be fixed into the rafters. The collar ties aren't there to stop the walls from spreading, they are there to stop the rafters from spreading. All the strength in roofs comes from the triangular shape. Going off the information you've given the base of the triangle in your roof space is a little over 7 foot and moving it up by a foot would lead you to a triangle with a base of just over 4.5 foot. This is a huge difference and I wouldn't recommend it. I couldnt be sure without seeing it but it might be a possibility to run new joists sitting on the wallplates and fixed into the rafters to tie it in at the bottom then move the collar ties up higher into the roof space.


  • #4
    red sean
    Registered Users, Registered Users 2 Posts: 2,917 ✭✭✭red sean


    chris445 wrote: »
    If I'm reading this corrctly and your ceiling joists aren't sitting on the wallplate then they can't be fixed into the rafters. The collar ties aren't there to stop the walls from spreading, they are there to stop the rafters from spreading. All the strength in roofs comes from the triangular shape. Going off the information you've given the base of the triangle in your roof space is a little over 7 foot and moving it up by a foot would lead you to a triangle with a base of just over 4.5 metres. This is a huge difference and I wouldn't recommend it. I couldnt be sure without seeing it but it might be a possibility to run new joists sitting on the wallplates and fixed into the rafters to tie it in at the bottom then move the collar ties up higher into the roof space.
    I don't think you are!
    Rabbo gave good advice and I think you should do as he says.


  • #5
    papa john
    Registered Users, Registered Users 2 Posts: 17 ✭papa john


    red sean wrote: »
    I don't think you are!
    Rabbo gave good advice and I think you should do as he says.
    Yes you are correct,the ceiling joists are NOT sitting on the wall plate.They are in between the blocks in the walls.They would stop the walls from spreading though i think but as you said the tie is to stop the rafters from spreading.


  • #6
    baby fish
    Registered Users, Registered Users 2 Posts: 269 ✭✭baby fish


    papa john wrote: »
    Yes you are correct,the ceiling joists are NOT sitting on the wall plate.They are in between the blocks in the walls.They would stop the walls from spreading though i think but as you said the tie is to stop the rafters from spreading.


    Joists in this position will not stop the walls spreading.
    The tie is to stop the rafters spreading, but it is also to stop the walls from being pushed out, as without the tie the rafters will spread and push the walls out.

    In a typical roof the ceiling joist ties the rafters together at wall plate level, in the situation you described ( a picture would be helpful) I would expect to see an RSJ with a wall plate under the collar tie (both sides), RSJ should be built into the gable walls and supported along its length.

    The triangulation occurs in the portion of the roof from the collar tie up to the ridge, and the weight of the roof is placed on the RSJ's and then onto the walls. The remainder of the rafter, i.e from the RSJ to the fascia board cannot not spread as it is hanging from the top triangle, so the walls will not be pushed out and crack. ( sometimes a 9 X 3 timber purlin can be used instead of an RSJ if there are enough internal walls to support where the 9X3 is jointed.

    Hard to say exactly without a picture but I wouldn't move the collar ties

    Hope this helps!!


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